A conventional method of pretreatment of non-conductive polymer surfaces prior to electroless metallization, typically electroless nickel plating or copper plating, includes etching the surface with chromium (VI) containing solutions followed by the activation with an ionic or colloidal solution of palladium compounds and either the reduction in sodium hypophosphite solution or acceleration in an acid solution such as sulfuric acid of palladium ions or colloidal palladium particles, respectively, adsorbed on the polymer surface.
Etching during a pretreatment step of the non-conductive substrate surface is required to obtain a hydrophilic and micro-roughened surface to allow sufficient quantities of palladium to adsorb on the surface and for ensuring proper binding of metal coatings to the non-conductive polymer surface. The activation with subsequent reduction or acceleration is performed in order to initiate the electroless deposition of the metal on the polymer. Thereafter, electroless plating with metal in the metallization solution takes place through auto-catalytic reaction where the metal deposited on the surface acts as a catalyst for electroless metal plating. Typically, electrolytic metal plating is performed on the first metal layer. Various metals can be applied such as chromium, nickel, copper, brass and other alloys of the foregoing metals.
Typically, polymer surfaces are treated with chromium (VI) containing pickling solutions which can be divided into solutions with high and low chromic acid content. For example such solutions which are based on chromium-sulfuric acid with high chromic acid content can include chromium (VI) oxide from 200 g/L to 550 g/L and sulfuric acid from 200 g/L to 500 g/L. Solutions with low chromic acid content contain less than 100 g/L chromic acid but the sulfuric acid content is at least 500 g/L.
The primary problems of the conventional method are related to the carcinogenic nature of the chromic acid solutions. Furthermore, low chromic acid containing etching solutions are prone to cause some metal deposition during electroless metallization, for example nickel, on the plating tools insulated with plastisol used in the metallization process. This can cause undesired plating with subsequent metal layers on the rack as well as contamination of subsequent plating by dissolving electroless metal layers from the rack.
Various methods to avoid the use of the carcinogenic chromic acid in the pretreatment of polymers have been suggested. U.S. 2005/0199587 discloses a method of etching non-conductive polymer surfaces in an acidic solution containing 20 g/L to 70 g/L of potassium permanganate. Optimal potassium permanganate concentration of the above mentioned solution is close to 50 g/L. When the concentration is below 20 g/L, the solution is ineffective with the upper concentration limit determined by the solubility of potassium permanganate. The etching is followed by the activation in a palladium solution containing amine and by further treatment in borohydride, hypophosphite or hydrazine solution.
The method, however, has substantial shortcomings. At high permanganate concentrations of around 50 g/L with phosphoric acid at around 48% v/v the etching solution quickly decomposes, especially at high temperatures of around 37° C. Often the solution has to be replenished with permanganate. Furthermore, insoluble permanganate decomposition products are formed contaminating the surface being metallized.
Etching in the permanganate solutions activates the plastisol surface of plating tools as it is coated with the product of the etching solution, i.e., manganese dioxide. The later stimulates adsorption of palladium compounds on the plastisol which tends to metallize in the electroless metal plating solutions. Formation of manganese dioxide on surfaces is characteristic of the permanganate etching solutions of any composition.
Accordingly, there is still a need for a method of inhibiting metallization of plastisol coated plating tools during electroless metal plating.